However McIlroy would not have finished the day were it not for a fine display of sportsmanship from Mickelson and Fowler. The pair were in the group ahead and allowed him to play up behind them as he tried to beat the fading light following the rain delay, then waited on the side of the 18th green to applaud the new champion.

Flashlights illuminated the night sky as McIlroy showed sheer relief after holing out his final putt, fist-pumping the air and celebrating on the course with his father Gerry before receiving the huge Wanamaker Trophy.

McIlroy's resurgence on the back nine capped an enthralling final day and saw him become the third youngest player of the modern era - after Tiger Woods and Jack Nicklaus - to win four of golf's biggest prizes, while he also becomes the first man to win back-to-back majors since Ireland's Padraig Harrington in 2008.

Heavy downpours threatened to take the championship into a fifth day

An ever-changing leaderboard saw an early five-way tie - between Fowler, Mickelson, McIlroy, Stenson and Bernd Wiesberger - transform into a narrow advantage for 25-year-old Fowler around the turn.

The competition then became a see-saw battle with Fowler, playing partner Mickelson, Stenson and McIlroy all taking a share of the lead over the back half.

Having fallen three shots behind Fowler after six holes, he swung the momentum back in his favour with a majestic eagle three at the 590-yard 10th.

BBC Sport golf correspondent Iain Carter

"Of his four majors, this was McIlroy's most impressive. He won the first two by eight strokes and took down a classy field at the Open for his third. In this one he demonstrated an astonishing will to win to come from three strokes back on the inward half. His eagle at the 10th showed him capable of golf beyond the rest of the field - no one else hit that par five in two strokes. It was the most thrilling major in recent years - McIlroy prevailed in near darkness and took a giant step towards golfing greatness."

That moved him back to within a shot of Fowler, and he claimed a share of the lead with another birdie on the 13th. And when Mickelson bogeyed the 16th, McIlroy regained the outright lead for the first time since he walked off the second green.

From there, despite reduced visibility as darkness began to fall at the Kentucky course, it was McIlroy's title to lose.

Rory McIlroy raced up the 18th hole to beat the fading light, sealing victory at 8.45pm local time

With the pre-tournament favourite taking a lead into the final day, albeit a narrow one, he was strongly fancied to win a second US PGA title after his success at Kiawah Island two years ago.

However, there had been signs during Saturday's third round that McIlroy was not in the same imperious form that saw him ease to victory at the Open last month.

A stoic McIlroy insisted the rain delay "did not bother him" but he quickly found himself adrift once play resumed at 19:40 BST.

The deluge brought the early starters off the sodden course, forcing a one hour 50 minute delay as water flooded the greens and fairways.

Stat attack

The US PGA has the biggest prize fund - $10m (£6m) - of all four majors. Winner Rory McIlroy pockets $1.8m (£1.1m).

Runner-up Phil Mickelson, a five-time major winner, had not earned a top-10 finish in any competition this year.

McIlroy's Open and US PGA double plus German Martin Kaymer's US Open win mean it is the first time in the modern era that Europe has claimed three majors in the same season.

Rickie Fowler is only the third man to finish in the top five of all four majors in a season in the modern era. The American joins Jack Nicklaus (1971 and 1973) and Tiger Woods (2000, 2005).

Ernie Els suggested in the enforced break that play should have been postponed until Monday, but would have been glad that organisers did not heed his advice after making a blistering start.

A run of four straight birdies helped the South African turn in a five-under-par 30, and he gained another shot on 11 before his slim hopes of a fifth major were finally ended with a bogey on the 17th.